Kinematics - Acceleration in Two Dimensions. Stuck.

[mattstjean]

Hi, I am also having trouble with the hockey puck question.

A hockey puck rebounds from a board as shown in my diagram. The puck is in contact with the board for 2.5 ms. Determine avg acceleration of the puck over the interval.

V_i = 26 m/s V_f = 21 m/s

I tried the cosine law but I keep getting 44 m/s and not 18. I don't understand how you guys got 18 m/s. I've plugged it in at least 100 times as
<br /> v_t = \sqrt{v_1^2 + v_2^2 - 2(v_1)(v_2)cos136}<br /> = \sqrt{26^2 + 21^2 - 2(26)(21)cos136}<br /> =44<br /> =

Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:
<br /> V_x = V_B sin \theta + (-V_A cos \beta ) <br /> = 21 sin(22) - 26 cos(22)<br /> =-16<br />

and

<br /> V_y = V_B cos \theta + (-V_A sin \beta ) <br /> = 21 (cos22) + 26(sin22)<br /> = 29 <br />

I then tried to figure out
<br /> \Delta V ^2= \Delta V_x ^2 + \Delta V_y^2 <br /> = sqrt{16^2 + 29^2}<br /> = 33<br />

Using that I tried to get the average acceleration by:

<br /> A_av = \Delta V / \Delta T<br /> <br /> A_av = 33 / 2.5x10^-3<br /> A_av = 13.2x10^3<br />
and to find the angle I tried to do :

<br /> \phi = tan^-1 = 16/29<br /> \phi = 29degrees<br />

However, the answer in my book says that the average acceleration is 7.3x10^3 [7.5degrees North of West]
Any help would be amazingly appreciated. Thanks.

 

[Doc Al]

Because that wasn't working I then tried Vector Components and I can't get that to work either. I did:
<br /> V_x = V_B sin \theta + (-V_A cos \beta ) <br /> = 21 sin(22) - 26 cos(22)<br /> =-16<br />

You have a mix of sine and cosine. Only one is correct.

and

<br /> V_y = V_B cos \theta + (-V_A sin \beta ) <br /> = 21 (cos22) + 26(sin22)<br /> = 29 <br />

Again, a mix of sine and cosine.

Redo this.

 

[mattstjean]

You have a mix of sine and cosine. Only one is correct.

Again, a mix of sine and cosine.

Redo this.

I don't know how to redo it. In my textbook it used them both together in the y and x component vector subtraction. I took the equations right out of my text, Nelson Physics 12.

 

[Doc Al]

I don't know how to redo it. In my textbook it used them both together in the y and x component vector subtraction. I took the equations right out of my text, Nelson Physics 12.

I'm not sure what equations you are talking about.

Do this: What's the x-component of Vi? The x-component of Vf?

 

[Danrio]

I have the same book and I am stuck on the example on right before the quesion box you asked about. can you please explain the lawn mower example in pg 28. I AM VERY CONFUSED